Gasification systems for producing combustible gas (sometimes described as “synthesis gas” and hereafter referred to as “syngas”) from carbonaceous materials are known, for example, as described in U.S. patent application Ser. No. 10/704,095 filed 7 Nov. 2003 (published under No. US 2004/0107638), U.S. patent application Ser. No. 11/801,030 filed 8 May 2007 (published under No. US 2007/0266914), and U.S. Pat. No. 6,120,567 issued 19 Sep. 2000, each of which is incorporated by reference herein. Syngas typically comprises H2 and CO and other combustible and non-combustible materials. Carbonaceous materials suitable as feedstocks for gasification systems include biomass, coal, other fossil fuels, and the like. Non-limiting examples of biomass include: wood (in any suitable form including sawdust, shavings, pellets, chips, other wood residue and the like), municipal waste, sewage, food waste, plant matter or the like.
Syngas has numerous potential uses. Syngas may be burned in some systems designed to burn fossil fuels such as coal or natural gas without substantial modification of the systems. However, syngas typically has a lower heating value than natural gas. Therefore, the flame temperature typical of burning syngas is undesirably low for some applications.
It would be beneficial to be able to use syngas as an alternative to fossil fuels in devices requiring relatively high flame temperatures such as lime kilns. For example, for effective heat transfer in a lime kiln to promote calcination, the adiabatic flame temperature should be about 1750° C. (3180° F.) or higher.
The moisture content of carbonaceous feedstocks, such as biomass, available for gasification can vary, and affects the composition and heating value of the syngas produced. Gasification of feedstocks which have a high moisture content may produce syngas with a relatively low heating value.
Some feedstocks and gasification processes can produce syngas having a relatively high heating value. However, in such cases it is typical that the increased heating value is at least partly due to tars and other heavier fractions entrained in the syngas. Such heavier fractions tend to condense at relatively low temperatures (e.g temperatures below approximately 360° C. (680° F.)). Such condensation can be problematic, especially in situations where it is necessary to transport the syngas a significant distance from a gasifier where it is produced to a burner where it is used as a fuel.
It would be beneficial to provide apparatus and methods in which syngas combustion provides a heat source suitable for direct-firing of lime kilns or other devices requiring relatively high flame temperatures. Improved methods and apparatus for using syngas to fuel burners for direct-firing in other applications where the temperature requirement is not as high, for example for power boilers (where the burner typically operates in a range of about 1800 to about 2200° F.) or dryers would also be beneficial.